1. Field of the Invention
The present invention relates to a discharge lamp drive apparatus which drives discharge lamps used as a backlight for a liquid crystal panel, and a liquid crystal display apparatus.
2. Description of the Related Art
In recent years, with an increase in size of a screen of a liquid crystal panel, a plurality of backlight discharge lamps are used in one liquid crystal panel. As means for driving the discharge lamps, there are a scheme which uses one inverter circuit and transformers to drive one discharge lamp, and a scheme which uses one inverter circuit and transformers to drive a plurality of discharge lamps. For example, Patent literature 1 discloses the scheme which uses one inverter circuit and transformers to drive a plurality of discharge lamps.
Of the two schemes, according to the scheme which uses one inverter circuit and transformers to drive a plurality of discharge lamps, a decrease in the number of circuit components can reduce a cost and power consumption.
Meanwhile, various kinds of safety standards are required in the inverter circuit. For example, International Standard IEC60950 restricts a peak-to-peak value of a current which flows when a current-limiting test resistance of 2 kΩ is interposed between an output winding terminal of a transformer and a ground in order to avoid an electric shock accident.
Specifically, assuming that a peak-to-peak value of a current flowing through the current-limiting test resistance is α mA and a frequency of an output voltage is β kHz, the following expression must be achieved:if β>1,α≦0.7β and α≦70
In the scheme which drives one discharge lamp by using one inverter circuit and transformers, constituting one type of a current-limiting element by using a winding resistance of an output winding of a transformer and a leakage inductance can readily satisfy International Standard IEC60950.
On the contrary, in the scheme which drives a plurality of discharge lamps by using one inverter circuit and transformers, since a winding current of the transformer increases in proportion to the number of the discharge lamps, a current-limiting element having a large resistance value must be constituted by adjusting a winding of the transformer to increase a winding resistance value or by adjusting a resonance point to increase the leakage inductance in order to satisfy International Standard IEC60950, and a countermeasure for meeting the standard is not necessarily easy.
Further, since the winding current of the transformer increases, the winding resistance value and the leakage inductance must be reduced in terms of promotion of high efficiency.
Therefore, in the scheme which drives a plurality of discharge lamps by using one inverter circuit and transformers, it is very difficult to satisfy International Standard IEC60950.
Furthermore, when International Standard IEC60950 cannot be met, reinforced insulation must be applied to the transformer, which obstructs a reduction in cost and size of a product.
Patent literature 1: JP-A-6-267674